Hey everyone,
I've just started out doing the physical prototyping of a stepper motor driver I've been theorizing for a while. But since I'm a bit of an electronics newb, I quickly found myself stuck on an initial step I thought wouldn't need much planning. It's driving me INSANE. I was wondering if anyone here could provide me with insight:
I have a LM317t circuit setup like the "Adjustable Regulator with Improved Ripple Rejection" from the typical applications section of http://www.national.com/ds/LM/LM117.pdf using a 220 ohm resistor as R1 and 5k trimpot as R2 set to around 660 ohms to ideally provide a 5v logic supply.
The IC has an aluminum heatsink mounted on it and I've also placed a few resistors in series from the circuit's output to ground, ensuring it has over 10mA of load on it.
It provides 5v perfectly after connecting a cheap 10v dc wall wart (without an earth ground connector).
HOWEVER when If I feed it the + and - terminals from my 35v large toroid based linear unregulated power supply (that I plan to build my project around), the circuit outputs a constant value around 9v, regardless of whether or not I ground the - DC rail to the chassis (that the incoming AC ground is tied to).
If I tune the 5k trimpot while the circuit is powered up, the output voltage forms a bit of a parabolic curve with a minimum at around 9v: If I turn the resistor too low past a certain point, the voltage actually begins to steadily INCREASE. I avoided turning it in a direction that made the output more than 15v.
STRANGELY, if I leave the trimpot at 660 ohms (for the intended 5v output) the proper regulation begins to kick in AFTER I switch off the AC line to the power supply and the 2x22000 uF caps start to drain into the circuit. The LM317 output quickly drops to and stays at 5v for around 20-30 seconds before dropping slowly to 0.
I started using a multimeter to poke around the PCB of my power supply's bridge rectifier module in order to see if I was misunderstanding the outputs or something. There is a terminal labelled "c" (assuming it stands for "common") between the + and - terminals that I deduced as a floating line to a point halfway in between two equivalent ~1.6k bleed resistors that lie in series between the positive and negative DC output rails. While on, the measured voltage between the - terminal and "c" is ~-17v and between + and "c" it is ~+17v. Just for kicks I tried using "c" as the negative rail for the LM317 circuit and it actually worked perfectly. But since the current is limited by resistors, I don't plan on actually using it. Also, in another just for kicks moment I assumed "c" meant "tie me to chassis" so I tied it to the chassis/ground. Then BAM!! absolutely nothing changed.
So in summary: the voltage regulator circuit works if the voltage is 10v from a wall wart, 17v from the +/c of the large toroid PSU, or the 35v->0v transient voltage at +/- after powering down the PSU. It doesn't work right if it's connected in the same way while on.
I've been trying to figure this one out for a couple of days and I have the feeling I've overlooked something really stupid/simple. Are there some characteristics of my PSU that are incompatible with the LM317 circuit? Is there too much ripple in the PSU's output? Does the LM317 behave this way only if it's operating in some sort of failure mode? Is there some grounding issue? Did I overlook something on the LM317 datasheet? I'm very stumped and I guess slightly afraid of electrocuting myself if I continue blindly probing for the source of this problem.
I've just started out doing the physical prototyping of a stepper motor driver I've been theorizing for a while. But since I'm a bit of an electronics newb, I quickly found myself stuck on an initial step I thought wouldn't need much planning. It's driving me INSANE. I was wondering if anyone here could provide me with insight:
I have a LM317t circuit setup like the "Adjustable Regulator with Improved Ripple Rejection" from the typical applications section of http://www.national.com/ds/LM/LM117.pdf using a 220 ohm resistor as R1 and 5k trimpot as R2 set to around 660 ohms to ideally provide a 5v logic supply.
The IC has an aluminum heatsink mounted on it and I've also placed a few resistors in series from the circuit's output to ground, ensuring it has over 10mA of load on it.
It provides 5v perfectly after connecting a cheap 10v dc wall wart (without an earth ground connector).
HOWEVER when If I feed it the + and - terminals from my 35v large toroid based linear unregulated power supply (that I plan to build my project around), the circuit outputs a constant value around 9v, regardless of whether or not I ground the - DC rail to the chassis (that the incoming AC ground is tied to).
If I tune the 5k trimpot while the circuit is powered up, the output voltage forms a bit of a parabolic curve with a minimum at around 9v: If I turn the resistor too low past a certain point, the voltage actually begins to steadily INCREASE. I avoided turning it in a direction that made the output more than 15v.
STRANGELY, if I leave the trimpot at 660 ohms (for the intended 5v output) the proper regulation begins to kick in AFTER I switch off the AC line to the power supply and the 2x22000 uF caps start to drain into the circuit. The LM317 output quickly drops to and stays at 5v for around 20-30 seconds before dropping slowly to 0.
I started using a multimeter to poke around the PCB of my power supply's bridge rectifier module in order to see if I was misunderstanding the outputs or something. There is a terminal labelled "c" (assuming it stands for "common") between the + and - terminals that I deduced as a floating line to a point halfway in between two equivalent ~1.6k bleed resistors that lie in series between the positive and negative DC output rails. While on, the measured voltage between the - terminal and "c" is ~-17v and between + and "c" it is ~+17v. Just for kicks I tried using "c" as the negative rail for the LM317 circuit and it actually worked perfectly. But since the current is limited by resistors, I don't plan on actually using it. Also, in another just for kicks moment I assumed "c" meant "tie me to chassis" so I tied it to the chassis/ground. Then BAM!! absolutely nothing changed.
So in summary: the voltage regulator circuit works if the voltage is 10v from a wall wart, 17v from the +/c of the large toroid PSU, or the 35v->0v transient voltage at +/- after powering down the PSU. It doesn't work right if it's connected in the same way while on.
I've been trying to figure this one out for a couple of days and I have the feeling I've overlooked something really stupid/simple. Are there some characteristics of my PSU that are incompatible with the LM317 circuit? Is there too much ripple in the PSU's output? Does the LM317 behave this way only if it's operating in some sort of failure mode? Is there some grounding issue? Did I overlook something on the LM317 datasheet? I'm very stumped and I guess slightly afraid of electrocuting myself if I continue blindly probing for the source of this problem.